Link Between Chronic Pain and Aggression in Horses Identified

Anyone who's suffered from chronic aches and pains will likely say the ever-present irritation can put a damper on their day, possibly even making them cranky. But is the same true for horses? A group of French researchers recently completed a study on the topic and, for the first time, demonstrated an association between chronic discomfort or pain and aggressive behavior in horses.

"Several studies have implicated that experiencing acute pain can affect the social behavior of humans and animals in that the risk of exhibiting aggressive or violent behavior is increased," said equine behavior scientist Carole Fureix, PhD, of the University of Rennes in Rennes, France, and lead researcher on the study.

Researchers on one study noted that 70% of human patients with chronic pain expressed feelings of anger. However, studies evaluating the link between chronic pain and aggression are lacking in animals. So Fureix and colleagues set out to determine if chronic discomfort or pain in horses was associated with increased aggression.

The researchers observed 59 horses (44 geldings and 15 mares that were 5 to 20 years old; mostly French Saddlebreds) residing at three different riding centers in France. The horses completed five different standardized behavioral tests that are commonly used in human-horse relationship studies (see sidebar). Subsequently, an equine chiropractor assessed the degree of back pain (or absence thereof) based on bony and soft tissue palpation and stiffness.

Key findings of the study included:

73% of the horses were severely affected by vertebral problems, based on the chiropractor's evaluation (most commonly the sacral area [croup] appeared severely affected, followed by the thoracic [withers to mid-back], cervical [neck], and lumbar [mid-back to croup] spinal regions);

More than 75% of the severely affected horses showed aggressiveness or negative reactions toward humans (e.g., looking at the experimenter with ears laid back, threats to bite by stretching the neck, or approaching the experimenter with ears laid back) in one or more of the five behavioral tests; and

According to the authors, this is the first experimental study conducted in horses supporting the hypothesis that there is an association between chronic discomfort or pain and aggression in horses.

Fureix noted, "Human awareness of this association may well alter the perception humans have of 'bad tempered' animals. Chronic pain should not be overlooked as a cause of aggression in horses."

She added that future studies on interpersonal relations and models of aggression should include chronic pain or discomfort as a consideration.

The study, "Partners with bad temper: reject or cure? A study of chronic pain and aggression in horses," was published in the journal PLoS ONE in 2010. The full length study is available for free online through the journal's website.

Behavioral Tests

The following common behavioral tests were used to evaluate the horses in the current study:

A motionless person test (where a person entered the horse's stall and stood with his or her back against the closed door for five minutes, facing inward and looking at the ground);

An approach contact test (where a person entered the horse's stall and stood motionless 1.5 meters from the horse until he resumed eating forage, at which point the person came closer to the animal and tried to touch his neck. The test was stopped when the experimenter could stroke the horse's neck continuously for two seconds or after three unsuccessful trials);

A sudden approach test (where a person walking slowly along the barn aisle appeared suddenly at the closed door of the stall while the horse was feeding; the doors were solid wood on the bottom and wire gated on top);

A saddle test (which followed the same procedure as the sudden approach test, except that the person carried a saddle on their right arm and opened the stall door); and

A halter fitting test (where a person entered the stall, holding a halter and approached the animal, walking slowly and regularly toward the horse's left shoulder, then put his or her right arm over the horse's neck and fitted the halter).

About the Author

Stacey Oke, MSc, DVM, is a practicing veterinarian and freelance medical writer and editor. She is interested in both large and small animals, as well as complementary and alternative medicine. Since 2005, she's worked as a research consultant for nutritional supplement companies, assisted physicians and veterinarians in publishing research articles and textbooks, and written for a number of educational magazines and websites.

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